1. Field of the Invention
The present invention relates to a deposition mask and a mask assembly having the same, and more particularly, to a deposition mask, which can achieve an improvement in deposition efficiency of organic matter in a process of forming an organic layer of an organic light emitting display device, and a mask assembly having the same.
2. Discussion of the Related Art
Recently, the increasing prominence of information technology has led to advances in display technology for visually displaying electrical information signals. Accordingly, a variety of flat panel displays having superior performance including slim design, low weight and low power consumption have been developed and rapidly replaced conventional Cathode Ray Tubes (CRT).
Representative examples of flat panel displays may include a Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Field Emission Display (FED), Electro Luminescent Display (ELD), Electro-Wetting Display (EWD), and Organic Light Emitting Diode (OLED) display.
Of the aforementioned displays, an organic light emitting diode (hereinafter, referred to as “OLED”) displays an image using organic light emitting diodes. An OLED is designed to generate light of a specific wavelength by exciton energy generated by recombination of electrons and holes. Such an OLED has advantages including superior display characteristics, such as high contrast ratio and fast response time, and easy realization of a flexible display, and, it may be classed as such an ideal next generation display.
In a general OLED, an active area in which a plurality of subpixels is arranged in a matrix and a remaining area, referred to as an inactive area, are defined. Each subpixel includes a thin-film transistor and an organic light emitting diode. The organic light emitting diode includes a first electrode, an organic layer, and a second electrode. The organic layer includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. The OLED having the above described configuration displays an image by applying a voltage of several volts to the first electrode and the second electrode. Thereby, current passing through the organic layer induces emission of light. That is, the OLED displays an image using the principle of emitting light using remnant energy which is generated by an exciton falling back to a ground state. The exciton is generated by recombination of hole and electron injected from the first electrode and the second electrode.
Meanwhile, in an organic layer forming process, a mask assembly is used to form light emitting regions corresponding to the subpixels. In this case, the mask assembly includes a frame coupled to the deposition mask and a deposition mask which is formed of a metal or plastic thin film and includes an aperture area corresponding to the active area and an intercepting area outside of the aperture area. In the mask assembly, the deposition mask is flat in an unfolded state thereof and is coupled to the frame via, e.g., welding. The frame is configured to maintain the flat state of the deposition mask.
To achieve improved yield by simultaneously manufacturing a plurality of organic light emitting displays, or to increase a size of an organic light emitting display, a size of a substrate is gradually increasing. This necessitates an increase in the size of the mask assembly to correspond to the substrate.
As described above, when a single deposition mask constitutes a large mask assembly, the deposition mask should have a large size. Therefore, even if the deposition mask is coupled to the frame in a stretched state, the deposition mask may sag under the weight. This sagging deposition mask may not come into close contact with the substrate, thereby making it difficult to perform deposition of organic matter according to a designed pattern. Moreover, if excessive tensile force is applied to the deposition mask to prevent the sag phenomenon, the tensile force may deform a pattern of the deposition mask, making it difficult to perform deposition of organic matter according to a designed pattern.
To solve the above described problems, constituting a large mask assembly corresponding to a large substrate using a plurality of deposition masks (hereinafter, referred to as “split deposition masks”) has been attempted. Specifically, in the mask assembly, a plurality of split deposition masks is consecutively arranged in parallel and is coupled to the frame via, e.g., welding. In this case, each of the plurality of split deposition masks has a relatively small size, thereby being free from sagging. However, gaps may occur at boundaries between the split deposition masks, and organic matter may pass through the gaps, thereby being unintentionally deposited outside the light emitting regions. This may deteriorate deposition efficiency of organic matter in a subsequent organic matter deposition process, and also, may deteriorate uniformity of an organic light emitting display. Therefore, the mask assembly including the plurality of split deposition masks needs to shield the gaps.